The present invention relates to a spindle motor which may be employed to drive, for example, a movable part of office automation equipment. More particularly, the present invention relates to a spindle motor which has high rigidity and a long lifetime.
With the achievement of hard disk drives having a high storage capacity and low power consumption, demand has been made for improvements in the performance of the spindle motors used to drive the hard disks. For example, spindle motors having a high rigidity and a long life have been demanded.
Conventional spindle motors of the type described above generally employ ball bearings. Ball bearings involve fundamental problems, such that they require lubrication by means of grease, and that it is difficult to improve the rotational accuracy and extend the lifetime thereof because the amount of axial vibration produced during rotation is relatively large. In order to solve these problems, employment of hydrodynamic bearings in place of ball bearings has been proposed.
FIG. 5 is a sectional view of a spindle motor employing hydrodynamic bearings, which was filed by the same applicant prior to this application (Japanese Patent Application No. 01-179647). As illustrated, this spindle motor comprises a motor stator 1 and a motor rotor 6.
The motor stator 1 comprises a base 2, a support shaft 3 that stands on the central portion of the base 2, a thrust bearing pad 4 that is secured to the base 2 and an annular radial bearing member 8 that is concentrically secured to the support shaft 3, and a stator 11 of a driving motor 5 secured to the support shaft 3 below the radial bearing member 8. A motor rotor 6 comprises a radial shaft sleeve 7 and a rotor 10 in the driving motor 5 secured inside a cylindrical member such that the radial shaft sleeve 7 faces the radial bearing member 8 and the rotor 10 faces the stator 11, together with a thrust bearing collar 9 that is secured to the lower end of the cylindrical member in opposing relation to the thrust bearing pad 4. The radial shaft sleeve 7 and the radial bearing member 8 face each other to form a herringbone groove hydrodynamic radial bearing, as shown in FIG. 6. The surface of the radial bearing member 8 that faces the radial shaft sleeve 7 is formed with grooves for generating dynamic pressure, for example, herringbone-shaped grooves C.sub.1.
The thrust bearing pad 4 and the thrust bearing collar 9 face each other to form a spiral groove hydrodynamic thrust bearing 12. The surface of the thrust bearing pad 4 that faces the thrust bearing collar 9 is formed with grooves for generating dynamic pressure, for example, spiral grooves C.sub.2 shown in FIG. 7.
The conventional spindle motor having the above-described arrangement suffers, however, from problems stated below. Hydrodynamic bearings, particularly hydrodynamic thrust bearings, need large starting torque because of the large contact resistance at the time of starting and are, in many cases, required to have excessive motor performance. When the number of revolutions increases, the thickness of the fluid layer in a hydrodynamic bearing increases, resulting in a lowering in the rotational rigidity. As a result, the axis of the rotor is inclined with respect to the support shaft, which leads to an increase in the starting torque because of local contact of the dynamic pressure surfaces. In addition, whirling of the shaft makes it impossible to obtain a satisfactory operating condition.